Wideband digital communications systems continue to increase in number and reach. Such systems, including the various digital subscriber lines (XDSL) (e.g., asynchronous, synchronous, very high speed), IEEE 802.11 a/g Wi-Fi Wireless local area networks (LAN), IEEE 802.16 WiMAX mobile area network (MAN), IEEE 801.20 Mobile Broadband Wireless Access (MBWA), and other wide and ultra wideband (UWB) communication systems, are being established in an effort to increase the accessibility of information and communication.
The information communicated over these systems, as well as the earliest forms of electronic systems, is typically modulated in order to transport a large amount of data over varying amounts of bandwidth. Single-carrier modulation schemes have generally been preferred in such systems as the single-carrier schemes tend to exhibit a higher data rate than multi-carrier schemes. One modulation scheme that has recently been considered for use in UWB systems is orthogonal frequency division multiplexing (OFDM).
OFDM is a digital, multi-carrier modulation scheme that uses a large number of closely-spaced orthogonal sub-carriers, each modulated with a conventional modulation scheme, such as quadrature amplitude modulation (QAM), at a low symbol rate. This combination has generally produced data rates on par with conventional single-carrier schemes. One of the primary advantages of OFDM over single-carrier schemes, however, is the ability to deal with severe channel conditions, such as multipath and narrowband interference, without the necessity for complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal.
Multiband OFDM (MB-OFDM) is one of the modulation schemes being supported and used in digital UWB communications systems, including wireless personal area network (WPAN) technology. One of the challenges in providing MB-OFDM to digital communications systems is the design and development of MB-OFDM UWB transceivers. Because the sub-carriers in MB-OFDM are orthogonal, cross-talk between the sub-channels is essentially eliminated. Thus, inter-carrier guard bands are not typically required. This feature greatly simplifies the design of the transmitters, receivers, and transceivers. However, MB-OFDM typically encourages very accurate frequency synchronization between the receiver and transmitter. Virtually any deviation from the sub-carrier can destroy the orthogonality of the sub-carriers and cause inter-carrier interference (ICI). OFDM is generally susceptible to Doppler shift during movement. Therefore, the compensation for Doppler shift and other factors that affect the orthogonality of the sub-carriers increase the complexity of the transmitters, receivers, and transceivers.
UWB transmission regulations request that the RF signal be transmitted within band groups. Currently, the multi-band systems divide the UWB spectrum into several smaller bands with some systems grouping those bands into band groups. For example, WiMedia Alliance's WIMEDIA® standard, which is the de facto standard radio platform for UWB wireless networking, divides the UWB spectrum into 14 bands (labeled as band #1-#14) organized into five band groups (labeled band group (or “BG”) #1-#5) over a center frequency range from 3432 MHz to 10296 MHz.
In meeting the requested band/band group organizational structure, the RF carrier quickly hops between each band within a particular band group. Two circuit architectures have been commonly deployed for MB-OFDM UWB transceivers. The first uses three different phase-locked loops (PLLs) operating at three different frequencies. The particular reference frequency is selected using an RF switch that selects the particular PLL.
There are several disadvantages to the three-PLL transceiver. First, PLLs occupy a considerable amount of chip/silicon space and consume relatively large amounts of power. Additionally, if the PLLs are not properly isolated from one another, each will introduce interference with the others. The RF switch also causes potential problems by introducing a different load impedance to the PLLs, which tends to disturb the lock status of the PLLs. This disturbed lock status typically calls for the addition of PLL output buffers that consume even more power and take up additional chip space.
The second transceiver architecture deploys a single PLL at a center frequency along with a complex frequency translation circuit that either shifts the PLL output frequency up or down by a certain amount or does not shift the frequency at all. Each state depends on which frequency band is needed. This architecture reduces the circuit chip area and also the power consumption compared with the three-PLL transceiver. However, the output of this transceiver contains certain inevitable harmonics with amplitudes between −30-−50 dBc, compared with typical harmonic amplitudes between −60-−70 dBc in WLAN or Global System for Mobile Communications (GSM) synthesizer output. Such higher spurious signals in the local oscillator (LO) output spectrum may cause several drawbacks to radio transceiver performance.
First, the higher spurious signals in the LO spectrum renders the receiver more susceptible to interference from external signals, such as those from WLAN in the 2.4-5 GHz band as well as the future signals from WiMAX in the >3.5 GHz band. Such high power interference signals are 40-50 dB higher than UWB signals to begin with, and when mixed into the MB-OFDM baseband spectrum, would likely disable normal operation of a UWB system without extra RF filtering.
Another drawback to the second transceiver architecture is the electromagnetic interference (EMI) that is transmitted from such a transceiver. In operation, the EMI of the transmission spectrum directly exports the spurious content with the intended UWB transmission. The additional spurious content and power level in the EMI could exceed the regulatory limits for EMI in any given location.